The invention relates to an artificial knee joint comprising a meniscus part which is displaceably journalled on a tibia platform and comprising a guiding part which is rotatably journalled relative to the tibia platform and which engages into a guide of the meniscus part, with the guiding part and the meniscus part having lateral guiding surfaces for guiding the meniscus part.
EP-A-0 529 408 discloses a meniscus part journalled on a tibia platform and guided by a rotatable guiding part. In this a rectilinear longitudinal guiding arises through parallel guiding surfaces of the guiding part and the meniscus part, onto which the rotation of the guiding part can be superimposed. It is usual to manufacture the contacting guiding surfaces of different materials such as for example polyethylene and metal in order to produce favorable sliding pairings. In plastics such as polyethylene it must be observed that the specific pressure forces do not become too large in order to prevent cold flow and abrasion.
In knee prostheses of the above-named kind the greatest lateral forces to be transmitted between the guiding part and the meniscus part arise in the extended position, in which the meniscus part assumes a forward position on the tibia platform. Only in this approximately extended position can a prosthesis wearer carry heavy loads analogously to the natural joint, push itself laterally away or receive blows which are not resiliently taken up by muscles and bands. At the same time, in its most forward position the meniscus part is at the greatest distance from the point of rotation of the guiding part, so that unfavorable conditions for a lateral force transmission can arise.
It is an object of the invention to provide guiding systems with a guiding part and meniscus in which excessive surface pressures under lateral forces are avoided. This object is satisfied in that the lateral guiding surfaces of the guiding part and the associated guiding surfaces of the meniscus part have different radii of curvature R1, R2 which are greater than 10 cm, with the sum of the reciprocals 1/R1+1/R2 being less than 0.2 cmxe2x88x921.
An advantage of the invention consists in that the point of rotation M for the guiding part can constructionally be placed relatively far towards the posterior in the direction of the cross bands which are present at the natural joint without impermissible tension peaks arising on account of this in the event of lateral forces.
Since contact points A, B between the guiding part and the meniscus part are provided with radii of curvature of more than 10 cm in the guiding direction, all possible curvature combinations yield a more favorable value than a constellation such as is for example shown in FIG. 1 as prior art.
The influence of the curvature can be shown using the example of spherical surfaces with sphere diameters D1, D2. If at a support point only the diameter of the sphere surface is changed, the maximum surface pressure is
a) in the case of a xe2x80x9csphere with diameter D on a planar surfacexe2x80x9d proportional to       1          D      2        3
b) in the case of xe2x80x9csphere D2 against sphere D1xe2x80x9d proportional to       1                  (                              D1            xc3x97            D2                                D1            +            D2                          )            2        3
c) in the case of xe2x80x9csphere D2 against larger spherical shell D1xe2x80x9d proportional to       1                  (                              D1            xc3x97            D2                                D1            -            D2                          )            2        3
If a theoretical comparison is made on this basis, there results for a sphere with a diameter of 1 cm which corresponds approximately to the width of a guiding part, in case a) a value of 1, in comparison with which stand a value of 0.13 with a sphere of 20 cm, and in case c) a value of 0.126 with a sphere of 20 cm against a shell of 200 cm diameter, which would correspond to an improvement by a factor of 7. Even in case b) two spherical surfaces with diameter 20 cm and 200 cm would still yield a value of 0.14, which corresponds to an improvement by a factor of 6.9.
From the point of view of the surface pressure, radii of curvature which are as large as possible are advantageous. In practice, however, limits are set in that the contact points must lie in the region of the overlapping guiding surfaces. The larger the radii of curvature are, the greater is the influence of the manufacturing tolerances and of the clearance between the guiding part and the meniscus on the contact points A, B still being located within the overlapping guiding surfaces with predetermined radii of curvature. For manufacturing reasons therefore those solutions are advantageous in which the guiding surfaces can be manufactured simply and within narrow tolerances, since the clearance depends on the precision with which the guiding surfaces of the meniscus lie at a predetermined distance from one another and the guiding surfaces of the guiding part lie at a predetermined distance from one another. With higher precision of the predetermined distances the smaller of the radii of curvature R1, R2 can take on values greater than 20 cm, and the sum of the reciprocal values of the radii of curvature 1/R1+1/R2 can be less than 0.1 cmxe2x88x921. With very high precision of the predetermined distances the smaller of the radii of curvature R1, R2 can take on values greater than 30 cm, and the sum of the reciprocal values of the radii of curvature 1/R1+1/R2 can be less than 0.067 cmxe2x88x921. In this consideration, planar guiding surfaces are associated with infinitely large radii of curvature.
An embodiment which is more simple in manufacturing technique results with a meniscus part of which the guiding surfaces extend straight and parallel to one another and a guiding part with convex guiding surfaces. Instead of straight, the guiding surfaces of the meniscus part can also extend slightly concavely with a substantially greater radius of curvature than that of the guiding part.
A further embodiment, which is considered more difficult in manufacturing technique, has a guiding part with guiding surfaces that extend straight and parallel to one another and a meniscus part that has convex guiding surfaces. Instead of straight, the guiding surfaces of the guiding part can also extend slightly concavely with a substantially greater radius of curvature than that of the meniscus part.